Description of the Related Art
The invention concerns a process for detecting measured substances in an ambient substance, especially for detecting gaseous warfare agents in ambient air, in which light reaction ions are first generated from a reaction substance and are added to the mixture of measured substance and ambient substance, in such a way that reaction ions in a spatially inhomogeneous distribution in a measurement chamber attach themselves to heavy molecules of the measured substance to form molecular ions; and that in the measurement chamber, an electric field that varies over time, alternates about a zero line, and has a predefined basic frequency and amplitude, is generated, the molecular ion current resulting from the electric field is measured, and the measured signal is analyzed.
The invention further concerns a device for detecting measured substances in an ambient substance, especially for detecting gaseous warfare agents in ambient air, with a measurement chamber, with first means for introducing the mixture of measured substance and ambient substance into the measurement chamber; with second means for generating light reaction ions from a reaction substance and for adding the reaction ions to the mixture; with third means for attaching the reaction ions to heavy molecules of the measured substance in the measurement chamber; with fourth means for generating in the measurement chamber an electric field which alternates a bout a zero line and has a predefined basic frequency and amplitude, with the fourth means being configured as an electrode pair in the measurement chamber and as a voltage source connected to the electrode pair; and with fifth means for detecting and analyzing the molecular ion current caused by the electric field in the chamber.
The closest known prior art device is disclosed in the article entitled "Detection of Chemical Warfare Agents by Means of an Ionization Chamber Operated in AC Mode," by S. Milinkovic et al., in "Proc. 3rd Int. Symp. Protection Against Chemical Warfare Agents", Umea, Sweden, Jun. 11-16, 1989.
In many areas of environmental protection and military engineering, it is desirable to be able to detect certain measured substances, preferably hazardous measured substances, in an environment consisting of air, water, or the like. If extremely dangerous measured substances, for example gaseous warfare agents, are to be detected in this context, not only are high detection sensitivity and reliability desirable, but stringent demands are also made in terms of response speed. This is especially true when it is not known if the measured substances in question are present in the relevant environment at all, since constant monitoring for the occurrence of these measured substances then takes place, and an alarm must be triggered as quickly as possible if the occurrence of these measured substances is observed.
In the area of military engineering, this applies especially to the area of toxic gases, which are known in a variety of compositions and effects.
For example, toxic gases are essentially differentiated into vesicants on the one hand, and nerve poisons on the other. While vesicants act on the human skin, where they cause erosion and burning, nerve poisons act on the human nervous system, leading to respiratory paralysis and the like.
The best known of the vesicants are the mustard gases, in this case especially the 2,2'-dichlorodiethyl sulfide known as sulfur mustard, with the structural formula: EQU Cl--CH.sub.2 --CH.sub.2 --S--CH.sub.2 --CH.sub.2 --Cl
and the tris(2-chloroethyl) amine with the structural formula: ##STR1##
Among the nerve poisons, the most important are the dimethylphosphoramidocyanidic acid ethyl ester known as Tabun, with a molecular weight of 162 amu and the following structural formula: ##STR2## and the O-isopropyl or -pinacolyl ester of methylphosphonofluoridic acid known as Sarin or Soman, with molecular weights between 140 and 182 amU and the following structural formula: ##STR3##
Last to be mentioned of the gaseous warfare agents are the phosphorylthiocholines known generally as V agents, with molecular weights of 267 amU and the following structural formula: ##STR4## especially "VX", with an even higher molecular weight and the following structural formula: ##STR5##
A variety of processes and devices for detecting these chemical warfare agents are already known. Some of these known processes and devices are based on the detection of mobile ions.
For example, the article by Milinkovic mentioned earlier describes a coaxial ionization chamber in which, in a cylindrical housing, a radial inlet tube and a radial outlet tube opposite it are provided for a gas mixture. For this purpose, gas is drawn in from the environment and sucked in through the inlet tube into the measurement chamber, being then discharged from it through the outlet tube once the measurement has been made.
The known measurement chamber uses a cylindrical electrode arrangement with an axial inner electrode and an outer electrode arranged on the cylindrical enveloping surface, with the gas flow under investigation flowing through the gap between the two electrodes. A radiation source that also influences the volume through which the gas flows is located at the axial end of the measurement chamber in a radial plane.
In the known device, ambient air is then pumped through the measurement chamber at a constant flow rate, and ionized by the radioactive source. The result of this is to form "reaction ions" from the ambient air (O.sub.2, N.sub.2) in combination with atmospheric moisture (H.sub.2 O). Specifically, this occurs as follows:
First the ambient air (O.sub.2, N.sub.2) is ionized under the action of .beta. radiation, forming O.sub.2 + and N.sub.2 + ions and free electrons e.sup.-.
If we now consider the N.sub.2 + ions, for example, these first bind to a nitrogen molecule 2N.sub.2 to form N.sub.4 + and N.sub.2. The N.sub.4 + in turn reacts with atmospheric moisture (H.sub.2 O) to form 2N.sub.2 and H.sub.2 O.sup.+. The H.sub.2 O.sup.+ in turn reacts with the H.sub.2 O in the atmospheric moisture to form H.sub.3 O.sup.+ and OH, which corresponds to a reaction between (H.sub.2 O)H.sup.+ and H.sub.2 O to form the positive reaction ions (H.sub.2 O).sub.8 H.sup.+. This positive reaction ion has a weight of 145 amU and a mobility of 2 cm.sup.2 /Vs.
Similarly, it can be shown for the negative reaction ions that the O.sub.2 and the free electrons e.sup.- ultimately react with atmospheric H.sub.2 O to form (H.sub.2 O).sub.6 O.sub.2, the negative reaction ion. This has a weight of about 140 amU and a mobility also on the order of 2 cm.sup.2 /Vs.
The positive and negative reaction ions explained above now react with the molecules of any gaseous warfare agents that may be present. If the molecules of the nerve poisons (organophosphorus compounds) are referred to as M.sub.A and the molecules of the vesicants, especially sulfur mustard, as M.sub.B, the following reactions can then be written: EQU (H.sub.2 O).sub.8 H.sup.+ +M.sub.A .fwdarw.M.sub.A (H.sub.2 O).sub.6 H.sup.+ +2H.sub.2 O EQU M.sub.A (H.sub.2 O).sub.6 H.sup.+ +M.sub.A .fwdarw.M.sub.A2 H.sup.+ +6H.sub.2 O
with the the formation of M.sub.A3 H.sup.+ also possible at higher concentrations of organophosphorus compounds.
With the vesicants, the corresponding reaction is as follows: EQU (H.sub.2 O).sub.6 O.sub.2.sup.- +M.sub.B .fwdarw.M.sub.B (H.sub.2 O).sub.4 O.sub.2.sup.- +2H.sub.2 O
The quasi-molecular ions or product ions formed in this manner have, in the case of the organophosphorus compounds, a mobility on the order of 1.5 to 0.5 cm.sup.2 /Vs at a molecular weight in the range between 250 and 700 amu, while for the vesicants (sulfur mustard), the ion mobility is approximately 1.5 cm.sup.2 /Vs with a molecular weight of 250 amu.
The above explanation shows the expected result, namely that the reaction ions are considerably lighter in weight and faster in terms of mobility than the product ions or quasi-molecular ions, with considerably higher molecular weights along with lower mobility.
For the sake of simplicity, the abbreviations "H.sup.+ " and "O.sub.2 " for the reaction ions, and the abbreviations "M.sub.A +" and "M.sub.B -" for the quasi-molecular ions, will be used in the description which follows.
If the gas flowing through the measurement chamber now also contains measured substances, especially gaseous warfare agents, that have molecules with a considerably higher molecular weight, the aforementioned reaction ions will then attach themselves to the molecules of the measured substances.
The type of attachment--i.e. whether the positively charged protons or the negatively charged oxygen ions attach to the molecules of the measured substances--depends on the nature of the measured substances.
In the case of the gaseous warfare agents explained above, conditions are such that with the mustard gases, the negatively charged reaction ions attach to the mustard molecules, while with the nerve poisons (Tabun, Sarin, Soman, V agents, and especially VX), a proton transfer occurs from the positively charged reaction ions to the warfare agent molecules. In the case of the latter reaction, it is also possible that in each case two molecules of the measured substance will bind a proton in pairs.
It is especially important in the present connection that with both types of gaseous warfare agents, namely with the vesicants (mustards) on the one hand and the nerve poisons (Tabun, Sarin, Soman, VX agents) on the other, differently charged molecular ions, namely negatively charged quasi-molecular ions for the vesicants and positively charged quasi-molecular ions for the nerve poisons, are formed in the respective cases.
It is also important, in the case of the known device, that the radioactive source is arranged so that it produces a spatially inhomogeneous distribution of reaction ions and thus also of quasi-molecular ions in the measurement volume of the measurement chamber. The inhomogeneity of the spatial distribution is configured so that on average, the distance from the quasi-molecular ions to one electrode is greater than the distance to the other electrode.
In the known device, an alternating current is then applied to the electrode arrangement. The resulting alternating electric field, i.e. a field oscillating about a zero line, then exerts a force on the quasi-molecular ions in the direction of one electrode during one half-wave, and in the direction of the other electrode during the other half-wave. However, since the average path lengths for the quasi-molecular ions to the two electrodes are, as mentioned, of different lengths, a greater ion current flows during one half-wave than during the other half-wave, since with an electric field of a suitably high frequency, the half-period is not sufficiently long to bring all the relatively heavy quasi-molecular ions to the respective target electrode.
The result, with the known arrangement, is therefore a direct-current component of the ion current. The sign of this direct-current fraction depends on which type of measured substance is present in the ambient air. If it is one of the aforementioned vesicants with negatively charged quasi-molecular ions, the sign of the direct-current component is positive, while with the nerve poisons it is negative.
With the known device, the result is therefore a positive or a negative output signal depending on the type of gaseous warfare agent detected; and it is known that this output signal can be further optimized in terms of amplitude by setting the frequency of the electric field appropriately.
The known device has the disadvantage, however, that it consistently produces measurement errors or even fails if what is present in the environment is not exclusively measured substances of a single type, but rather mixtures of measured substances of both types, for example vesicants and nerve poisons simultaneously.
On the other hand, it is of considerable advantage to be able to have available processes and devices with which, even in situations in which vesicants and nerve poisons are being used simultaneously, both the one and the other substance can be detected quickly and reliably, without having the signals from one substances compensate for the signals of the other substance.
A further problem that arises in the detection of measured substances of the type discussed here is that detection of these measured substances must be differentiated from interfering substances and other interfering factors that occur in a real-world environment.
If we once again consider the area of military engineering in this context, it may for example be necessary in a conflict situation to be able to detect gaseous warfare agents selectively, even when the ambient air is at the same time permeated by hydrocarbon vapors, for example leaking gasoline and diesel fuel, or by smoke or other organic compounds. In addition, detection must also be possible if fluctuations in pressure, temperature, or atmospheric humidity occur during the measurement. The interfering factors just mentioned can occur, for example, if the process is performed on board a vehicle or if the device is located on board a vehicle. If the vehicle is, for example, a helicopter, a measurement device will also be exposed to numerous interfering effects which must suppressed as much as possible when detecting these highly sensitive measured substances.
It is also a known process to use "tandem measurement chambers" to suppress the influence of interfering substances. Such tandem measurement chambers have an unaltered measurement chamber, for example of the type mentioned initially, and also have a second reference chamber in the gas inlet of which is arranged a filter that filters out the substances being measured. When measuring gaseous warfare agents, such a filter can be, for example, the usual filter for a gas mask.
When such a tandem measurement chamber is used in a measurement situation in which, for example, gasoline vapors are acting as an interfering factor, these gasoline vapors are not retained by the reference chamber filter and therefore permeate the measurement chamber and reference chamber simultaneously. Consequently, the readings from the two chambers change simultaneously, so that this type of reading can be eliminated as an interfering variable. On the other hand, if a gaseous warfare agent encounters the tandem measurement chamber, it permeates only the measurement chamber and not the reference chamber, since it is blocked off from the latter by the filter, so that a true reading occurs only in the measurement chamber.
In an alternative configuration, a filter can also be placed in front of a measurement chamber and then removed from it, so that one measurement chamber serves in succession as measurement chamber and reference chamber.
The known arrangements just described have the disadvantage, however, of being encumbered with a long time constant. For example, in reality it is impossible to obtain filters that ideally retain the measured substance and immediately allow free passage of all other substances. In practice, conditions are in fact such that a filter also presents at least some resistance to interfering substances, for example gasoline vapors, with the consequence that the reference chamber is slow to fill up with the interfering substance, while the latter has already penetrated into the measurement chamber without hindrance. To eliminate incorrect measurements, it is therefore always necessary to wait a certain amount of time for equilibrium to occur between measurement chamber and reference chamber. The times required for this in practice are on the order of minutes; but this period is too long for the detection of gaseous warfare agents. What are instead desired are reaction times on the order of, for example, 10 seconds, so that an alarm can be triggered before the suddenly occurring gaseous warfare agents have caused any damage.
The periodical "Isotopenpraxis," Volume 26, No 4, pages 176-180 (1990), discloses a mathematical method for using a Fortran program to find an iterative solution to differential equation systems that are applicable to the kinetics of various physical processes. These include, for example, an ionization gas detector, in which ions present in a measurement chamber are influenced by a pulsed electric field.
The article "New Ion Mobility Techniques" by C. Blanchard, published in "Proceedings of the 1987 U.S. Army Scientific Conference on Chemical Defense Research," Aberdeen Proving Ground, MD, CRDBC-Sp-88013, 1971 (April 1988) discloses an ion mobility spectrometer (IMS) in which non-linear electric fields are applied to the mobile ions. These non-linear electric fields are partly spatially and partly temporally non-linear, and are designed to raise the sensitivity of the spectrometer.
European Patent EP-A-O 253 155 discloses a field-regulating arrangement for an ion mobility spectrometer (IMS) in which a temporally non-linear electric field is also generated. However, the emphasis with this IMS, as with the IMS explained above, is on spatial separation of regions with differing field strength profiles. The possibility of separate detection of two measured substances with differently charged quasi-molecular ions is neither discussed nor possible in this connection.